Lithium tantalate substrate and process for its manufacture

ABSTRACT

A lithium tantalate substrate obtained by working in the state of a substrate a lithium tantalate crystal grown by the Czochralski method is buried in a mixed powder of Al and Al 2 O 3 , followed by heat treatment carried out at a temperature kept to from 350 to 600° C., to manufacture a lithium tantalate substrate having volume resistivity which has been controlled within the range of from more than 10 8  to less than 10 10  Ωcm. The substrate obtained has no pyroelectricity, and it can be made colored and opaque from a colorless and transparent state and also sufficiently has the properties required as a piezoelectric material.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of prior application Ser.No. 10/574,274 filed on Mar. 31, 2006 which was a §371 National StageApplication of PCT/JP2004/015189, filed on Oct. 7, 2004, the priorapplications being hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a lithium tantalate (LT) substrate used insurface acoustic wave devices and the like, and is more particularlyconcerned with improvements in an LT substrate which can not easilycause a lowering of yield in device fabrication processes and also hasproperties required as piezoelectric materials, and in a process formanufacturing the same.

BACKGROUND ART

Lithium tantalate (LT) crystals are ferroelectric materials having amelting point of about 1,650° C. and a Curie temperature of about 600°C. Then, LT substrates are chiefly used as materials for surfaceacoustic wave (SAW) filters used to remove signal noise of cellulartelephones.

It is predicted that a SAW filter having a frequency region of about 2GHz hereafter increases rapidly because, e.g., cellular telephones arebeing made high-frequency and Bluetooth (2.45 GHz), which is a cordlessLAN of electronic equipment of various types, has prevailed.

Such a SAW filter has a structure in which a pair of comb-shapedelectrodes are formed of metallic thin films of an AlCu alloy or thelike on a substrate constituted of a piezoelectric material such as LT.Such comb-shaped electrodes play an important role that governs thepolarity of devices. Also, the comb-shaped electrodes are made up byforming metallic thin films on the piezoelectric material by sputtering,thereafter leaving a pair of comb-shaped patterns, and removingunnecessary portions by etching using a photolithographic technique.

In order to make the filter adaptable to higher frequency, thecomb-shaped patterns must finely and also thinly be formed. In devicesof about 2 HGz in frequency, the filter has an electrode-to-electrodedistance of 0.3 to 0.4 μm which is about ⅓ of that of currentlyprevalent devices of 800 MHz in frequency, and a thickness of about 200nm or less which is ⅕ or less of the same.

The LT single crystal is, in an industrial scale, usually chieflyobtained by the Czochralski method, using an iridium crucible having ahigh melting point, and is grown in an electric furnace having anatmosphere of a nitrogen-oxygen mixed gas of several to about 10% inoxygen concentration, then cooled in the electric furnace at a statedcooling rate, and thereafter taken out of the electric furnace (seeAlbert A. Ballman, Journal of American Ceramic Society, Vol. 48, 1965).

The LT crystal thus grown is colorless and transparent or has a highlytransparent pale yellow color. After it has been grown, it is subjectedto heat treatment at an soaking temperature close to the melting pointin order to remove the residual strain produced by thermal stress of thecrystal, and further to poling treatment so as to be single-polarized,i.e., a series of treatment in which the LT crystal is heated from roomtemperature to a stated temperature of Curie temperature or more, avoltage is applied to the crystal, and, keeping the voltage applied, thecrystal thus heated is cooled to a stated temperature of Curietemperature or less, and thereafter, stopping the voltage being applied,cooled to room temperature. After the poling treatment, the LT crystalthe periphery of which has been ground in order to make the crystal havea proper shape (an ingot) is put to mechanical working such as slicing,lapping and polishing steps, and made into the LT substrate. The LTsubstrate obtained finally is substantially colorless and transparent,and has a volume resistivity of about 10¹⁴ to 10¹⁵ Ωcm.

Now, in the LT substrate obtained by such a conventional method, it maycome about that, because of sparks generated in a process of fabricatinga surface acoustic wave device when the substrate surface is charged updue to temperature changes undergone during the process, on account ofthe pyroelectricity that is a property of the LT crystal, patternsformed on the substrate surface are destroyed and further the substrateis broken to cause a lowering of yield in the device fabricationprocess.

There also arises a problem that, since the LT substrate has a highlight transmittance, the light transmitted through the interior of thesubstrate in the step of photolithography that is one step in the devicefabrication process reflects from the back of the substrate and returnsto the surface to make poor the resolution of the patterns formed.

Accordingly, in order to solve such problems, as disclosed in JapanesePatent Applications Laid-open No. H11-92147 and No. H11-236298, it isproposed to expose a lithium niobate (LN) crystal to a reducingatmosphere (stated specifically an atmosphere of a gas selected fromargon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide,oxygen, and combination of any of these) in the range of 500 to 1,140°C. to blacken a wafer of LN crystal so that the substrate can be keptfrom having a high light transmittance and also it can have a highelectrical conductivity to thereby keep the light from returning fromthe back of the substrate and at the same time make the substrate have alow pyroelectricity.

However, the invention disclosed in the publications Japanese PatentApplications Laid-open No. H11-92147 and No. H11-236298 is intended notonly for the LN crystal but also the lithium tantalate (LT) crystal, butthese publications Japanese Patent Applications Laid-open No. H11-92147and No. H11-236298 have substantially no disclosure at all as to the LTcrystal. Then, experiments made by the present inventors have confirmedthat the method disclosed therein is effective in respect of the lithiumniobate crystal, having a melting point of as low as about 1,250° C.,but not effective in respect of the LT crystal, having a melting pointof as high as 1,650° C.

Under such a technical background, the present inventors have alreadyproposed a method quite different from the method disclosed in thepublications Japanese Patent Applications Laid-open No. H11-92147 andNo. H11-236298, i.e., a method in which the LT crystal is buried in ametallic powder of a metal selected from the group consisting of Ca, Al,Ti and Si (what is called a reducing agent) and this is subjected toheat treatment at a temperature kept to 350 to 600° C. to manufacturethe lithium tantalate (LT) substrate (see the specification in JapanesePatent Application No. 2003-104176).

The LT substrate manufactured by this method, like the lithium niobate(LN) substrate disclosed in the publications Japanese PatentApplications Laid-open No. H11-92147 and No. H11-236298, can be keptfrom having a high light transmittance and also can have a highelectrical conductivity. Hence, in the lithium tantalate (LT) substrateas well, it enables solution of the above problems of a lowering ofyield in the device fabrication process and of making poor theresolution of the patterns formed.

However, the invention disclosed in the specification in Japanese PatentApplication No. 2003-104176, has a problem that, if the reducingconditions in respect of the lithium tantalate (LT) substrate are toostrong, the LT substrate obtained may have a very low pyroelectricityand hence the problem to be caused by the charge-up can be remedied, butthe LT substrate may also similarly have a low piezoelectricity to havelow properties required as a piezoelectric material, and has a problemthat, if on the other hand the reducing conditions in respect of thelithium tantalate (LT) substrate are too weak, it is difficult for theLT substrate obtained to have a low pyroelectricity. Thus, there hasbeen room for further improvement.

DISCLOSURE OF THE INVENTION

The present invention has been made taking note of such problems, andwhat the present invention regards as its subject is to provide alithium tantalate (LT) substrate which has solved the problem to becaused by the charge-up of the substrate and also sufficiently has theproperties required as a piezoelectric material; and a process formanufacturing such a substrate.

Accordingly, in order to settle the above subject, the present inventorshave continued their extensive studies. As a result, they havediscovered that, where the lithium tantalate (LT) substrate has volumeresistivity which has been controlled within the range shown below, thisLT substrate solves the problem to be caused by the charge-up andmoreover sufficiently has the properties required as a piezoelectricmaterial, and also that this LT substrate may be obtained by burying anLT crystal in a mixed powder of Al and Al₂O₃, followed by heat treatmentcarried out at a temperature kept to from 350 to 600° C.

That is, the lithium tantalate (LT) substrate according to the presentinvention is characterized by having volume resistivity which has beencontrolled within the range of from more than 10⁸ to less than 10¹⁰ Ωcm.

The process for manufacturing the lithium tantalate substrate accordingto the present invention is a process for manufacturing a lithiumtantalate substrate by using a lithium tantalate crystal grown by theCzochralski method, wherein a lithium tantalate crystal worked in thestate of a substrate is buried in a mixed powder of Al and Al₂O₃,followed by heat treatment carried out at a temperature kept to from 350to 600° C., to manufacture a lithium tantalate substrate having volumeresistivity which has been controlled within the range of from more than10⁸ to less than 10¹⁰ Ωcm.

In the lithium tantalate (LT) substrate according to the presentinvention, its resistivity is controlled within the range of from morethan 10⁸ to less than 10¹⁰ Ωcm. Hence, the substrate has nopyroelectricity, and it can be made colored and opaque from a colorlessand transparent state and also sufficiently has the properties requiredas a piezoelectric material. Therefore, it no longer comes about that,e.g., because of sparks generated when the substrate surface is chargedup due to temperature changes undergone during the process offabricating a surface acoustic wave device or the like, patterns formedon the substrate surface are destroyed and further the substrate isbroken. It also no longer comes about that the light transmitted throughthe interior of the substrate in the step of photolithography reflectsfrom the back of the substrate and returns to the surface to make poorthe resolution of the patterns formed.

BEST MODES FOR PRACTICING THE INVENTION

The present invention is described below in detail.

In the first place, the LT crystal changes in electrical conductivityand color depending on the density of oxygen holes (vacancies) presentin the crystal. Upon introduction of oxygen holes into the LT crystal,the valence of some Ta ions changes from 5+ to 4+ because of thenecessity of balancing charges, to produce electrical conductivity andat the same time bring about absorption of light.

The electrical conductivity is considered to be produced because theelectrons that are carriers move between Ta⁵⁺ ions and Ta⁴⁺ ions. Theelectrical conductivity of a crystal depends on the product of thenumber of carriers per unit volume and the mobility of carriers. If themobility is invariable, the electrical conductivity is proportional tothe number of oxygen holes. The change in color due to the absorption oflight is considered to depend on the level of electrons introduced bythe oxygen holes.

The number of oxygen holes may be controlled by heat treatment makinguse of what is called a reducing agent, utilizing the solid-to-solidequilibrium. Then, in the present invention, as the reducing agent forthe LT crystal, Al (aluminum) is used. Stated specifically, a materialLT substrate is buried in a mixed powder of Al and Al2O₃, in the stateof which the heat treatment is carried out.

The heat treatment may preferably be carried out in an atmosphere of aninert gas such as nitrogen gas or argon gas, or vacuum or the like, inorder to prevent deterioration due to any excess oxidation of the Al(aluminum) itself constituting the powder. Also, the temperature of theheat treatment may preferably be a high temperature. Its upper limit islimited to the Curie temperature of the LT crystal so that the LTsubstrate having been made single-polarized by the poling treatment maynot come multi-polarized.

As conditions most preferable taking account of the controllability oftreating steps, the properties of the substrate to be finally obtained,the uniformity of that properties, the reproducibility and so forth, itis effective to use as a material a wafer (material LT substrate) cutout of an LT crystal ingot having been subjected to poling, and bury thematerial LT substrate in the mixed powder of Al and Al₂O₃, followed byheat treatment carried out at the Curie temperature or below of the LTcrystal and in the atmosphere of an inert gas such as nitrogen gas orargon gas, or vacuum or the like. Incidentally, when carried out in anatmosphere of vacuum, the conditions for reduction may be too strong,and, when carried out in an atmospheric-pressure atmosphere of an inertgas, it takes a long time to effect the reduction. Hence, it is morepreferable for the heat treatment to be carried out in areduced-pressure atmosphere of an inert gas (such as nitrogen gas orargon gas).

The LT crystal also has strong bond ionic properties and hence the holesdiffuse at a relatively high rate. However, in-crystal diffusion ofoxygen is required for the change in the density of oxygen holes, andhence the crystal must be kept in an atmosphere for a certain time. Therate of such diffusion depends greatly on the temperature, and theoxygen hole density does not change in actual time in the vicinity ofroom temperature. Accordingly, in order that an LT substrate having thedesired properties is obtained in a short time, the material LTsubstrate must be kept at a temperature high enough to achieve asufficient oxygen diffusion rate, and in an atmosphere of a low oxygenconcentration.

After treatment at the high temperature, the LT substrate thus treatedmay immediately be cooled, whereby a crystal having kept the density ofoxygen holes introduced at the high temperature can be obtained at roomtemperature. The lower limit of treatment time may experimentally bedetermined with ease according to the treatment temperature in the aboveheat treatment process, taking account of economical advantages.

Now, the pyroelectric effect (pyroelectricity) is ascribable to thedeformation of lattices that is caused by changes in crystaltemperature. In crystals having electric dipoles, it is understood thatthe pyroelectric effect is produced because the distance between thedipoles changes by temperature. The pyroelectric effect is produced onlyin materials having a high electrical resistance. Electric charges areproduced in the direction of dipoles (the Z direction in the LT crystal)at the crystal surface as a result of ionic displacement. In materialshaving a low electrical resistance, such electric charges comeneutralized because of the electrical conductivity the crystal itselfhas. Then, in a usual transparent LT crystal, its volume resistivity isat the level of 10¹⁵ Ωcm as stated previously, and hence thepyroelectric effect develops remarkably.

However, in the lithium tantalate (LT) substrate according to thepresent invention, its volume resistivity is controlled within the rangeof from more than 10⁸ to less than 10¹⁰ Ωcm. Hence, the substrate has nopyroelectricity, and it can be made colored and opaque from a colorlessand transparent state and also sufficiently has the properties requiredas a piezoelectric material. Also, the colored and opaque tone of thelithium tantalate (LT) substrate according to the present inventionlooks dark brown through transmitted light, and looks black throughreflected light. Accordingly, this phenomenon of being made colored andopaque is herein called “blackening”.

As a practical method of judging whether or not the lithium tantalate(LT) substrate has no longer the pyroelectricity as the effect broughtby the heat treatment described above, a heat cycle test is availablewhich is conducted sumilating temperature changes the LT substrateundergoes in an actual process for fabricating surface acoustic wavedevices. More specifically, where a heat cycle in which a sample isheated from room temperature to 200° C. at a rate of 10° C./minute andthereafter cooled to room temperature at a rate of 10° C./minute isapplied to LT substrates, sparks are seen at the substrate surface inthe case of LT substrates obtained by the conventional method. On theother hand, no spark is seen at the substrate surface in the case ofblackened LT substrates. Accordingly, the judgement on whether or notthe blackening has taken place is effective as a practical method forthe judgement of LT substrates.

Incidentally, the blackening is clearly observable where the heattreatment is carried out for 4 hours or more, provided that, when viewedthrough transmitted light, the degree of coloring for the blackening islower where the heat treatment is carried out in the atmosphere of aninert gas (such as nitrogen gas or argon gas) than where it is carriedout in the atmosphere of vacuum, even though substrates have equalvolume resistivities. A substrate treated in the atmosphere of vacuum isstrongly colored at the substrate surface and the vicinity thereof.Thus, the oxygen hole density at the substrate surface and the vicinitythereof is presumed be high. The oxygen holes are also a sort of crystaldefects (imperfections), and hence, in view of mechanical strength ofthe substrate, it is more desirable that the desired volume resistivityis attained by relatively thin coloring.

The present invention is described below in greater detail by givingExamples.

Example 1

Using a raw material having congruent composition, an LT single crystalof 4 inches in diameter was grown by the Czochralski method. The crystalwas grown in an atmosphere of nitrogen-oxygen mixed gas of about 3% inoxygen concentration. An ingot of the crystal obtained was transparentand pale yellow.

This ingot of the crystal was subjected to heat treatment in order toremove thermal strain and to poling treatment so as to besingle-polarized, followed by periphery grinding, slicing and polishingto obtain a material LT substrate with 36° RY (rotated Y axis). Thematerial substrate obtained was colorless and transparent, and had avolume resistivity of 10¹⁵ Ωcm, a Curie temperature of 603° C. and asurface acoustic wave velocity of 4,150 m/second.

The material substrate obtained was buried in a mixed powder of 50% byweight of Al and 50% by weight of Al₂O₃ and then subjected to heattreatment at 350° C. for 20 hours in an atmosphere of nitrogen gas andunder a reduced pressure of 500 Torr.

The substrate having been subjected to the heat treatment was opaque anddark reddish brown (transmittance of light of 365 nm in wavelength, insubstrate: 58%) and had a volume resistivity of 9.30×10⁹ Ωcm.

Incidentally, the light transmittance was measured with aspectrophotometer (U-3400) manufactured by Hitachi Ltd., and the volumeresistivity was measured by the three-terminal method according to JISK-6911.

Next, on the substrate having been subjected to the heat treatment, aheat cycle test was conducted in which the substrate was heated fromroom temperature to 200° C. at a rate of 10° C./minute and thereaftercooled to room temperature at a rate of 10° C./minute. As the result, nosubstrate potential came about and no phenomenon of sparking was seen atall.

Further, the substrate obtained had a Curie temperature of 603° C. and asurface acoustic wave velocity of 4,150 m/second. Thus, the values ofphysical properties influencing the surface acoustic wave devicecharacteristics did not differ from those of the conventional product,the 36° RY substrate.

Example 2

The heat treatment was carried out under substantially the sameconditions as those in Example 1 except that the heat treatmenttemperature was 550° C., to obtain a substrate which was opaque and darkreddish brown (transmittance of light of 365 nm in wavelength, insubstrate: 55%) and had a volume resistivity of 1.80×10⁹ Ωcm.

This substrate showed heat cycle test results like those in Example 1,and also had properties such as Curie temperature like those in Example1.

Example 3

The heat treatment was carried out under substantially the sameconditions as those in Example 1 except that the heat treatmenttemperature was 600° C., to obtain a substrate which was opaque and darkreddish brown (transmittance of light of 365 nm in wavelength, insubstrate: 53%) and had a volume resistivity of 1.10×10⁸ Ωcm.

This substrate also showed heat cycle test results like those in Example1, and also had properties such as Curie temperature like those inExample 1.

Example 4

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that the material substrate was buried in a mixedpowder of 10% by weight of Al and 90% by weight of Al₂O₃ and that theheat treatment time was set for 40 hours.

The substrate obtained was opaque and dark reddish brown (transmittanceof light of 365 nm in wavelength, in substrate: 58%) and had a volumeresistivity of 9.80×10⁹ Ωcm.

This substrate also showed heat cycle test results like those in Example1, and also had properties such as Curie temperature like those inExample 1.

Example 5

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that the material substrate was buried in a mixedpowder of 10% by weight of Al and 90% by weight of Al₂O₃ and that theheat treatment temperature was set at 550° C. and the heat treatmenttime was set for 40 hours.

The substrate obtained was opaque and dark reddish brown (transmittanceof light of 365 nm in wavelength, in substrate: 55%) and had a volumeresistivity of 2.10×10⁹ Ωcm.

This substrate also showed heat cycle test results like those in Example1, and also had properties such as Curie temperature like those inExample 1.

Example 6

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that the material substrate was buried in a mixedpowder of 10% by weight of Al and 90% by weight of Al₂O₃ and that theheat treatment temperature was set at 600° C. and the heat treatmenttime was set for 40 hours.

The substrate obtained was opaque and dark reddish brown (transmittanceof light of 365 nm in wavelength, in substrate: 53%) and had a volumeresistivity of 1.40×10⁸ Ωcm.

This substrate also showed heat cycle test results like those in Example1, and also had properties such as Curie temperature like those inExample 1.

Example 7

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that the material substrate was buried in a mixedpowder of 75% by weight of Al and 25% by weight of Al₂O₃ and thensubjected to heat treatment at 550° C. for 40 hours in an atmosphere ofnitrogen gas and under atmospheric pressure.

The substrate obtained was opaque and dark reddish brown (transmittanceof light of 365 nm in wavelength, in substrate: 58%) and had a volumeresistivity of 2.30×10⁹ Ωcm.

This substrate also showed heat cycle test results like those in Example1, and also had properties such as Curie temperature like those inExample 1.

Example 8

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that it was carried out at 550° C. for 10 hours inan atmosphere of vacuum.

The substrate obtained was opaque and dark reddish brown (transmittanceof light of 365 nm in wavelength, in substrate: 43%) and had a volumeresistivity of 9.30×10⁸ Ωcm.

This substrate also showed heat cycle test results like those in Example1, and also had properties such as Curie temperature like those inExample 1.

Comparative Example 1

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that the material substrate was not buried in themixed powder of Al and Al₂O₃ and subjected to heat treatment at 1,000°C. for 40 hours in an atmosphere of nitrogen gas and under atmosphericpressure.

The substrate obtained was colorless and transparent and was not seen tohave been blackened (transmittance of light of 365 nm in wavelength, insubstrate: 71%) and had a volume resistivity of 1 to 2×10¹⁵ Ωcm.

On the substrate having been thus treated, a heat cycle test wasconducted in which the substrate was heated from room temperature to200° C. at a rate of 10° C./minute and thereafter cooled to roomtemperature at a rate of 10° C./minute. As the result, a phenomenon wasseen in which vigorous sparking occurred on the substrate surface.

Comparative Examples 2 and 3

The heat treatment was carried out in the same manner as the treatmentin Example 1 except that the material substrate was not buried in themixed powder of Al and Al₂O₃ and then subjected to heat treatment at800° C. (Comparative Example 2) or 480° C. (Comparative Example 3) for40 hours in an atmosphere of nitrogen gas and under atmosphericpressure.

The substrates obtained were colorless and transparent and were not seento have been blackened (transmittance of light of 365 nm in wavelength,in substrate: 72%) and had a volume resistivity of 1 to 2×10¹⁵ Ωcm.

On each substrate having been thus treated, a heat cycle test was alsoconducted in which the substrate was heated from room temperature to200° C. at a rate of 10° C./minute and thereafter cooled to roomtemperature at a rate of 10° C./minute. As the result, a phenomenon wasseen in which vigorous sparking occurred on the substrate surface.

POSSIBILITY OF INDUSTRIAL APPLICATION

As described above, in the lithium tantalate (LT) substrate according tothe present invention, its volume resistivity is controlled within therange of from more than 10⁸ to less than 10¹⁰ Ωcm. Hence, the substratehas no pyroelectricity, and it can be made colored and opaque from acolorless and transparent state and also sufficiently has the propertiesrequired as a piezoelectric material. Therefore, it no longer comesabout that, because of sparks generated when the substrate surface ischarged up due to temperature changes undergone during the process offabricating a surface acoustic wave device or the like, patterns formedon the substrate surface are destroyed and further the substrate isbroken. It also no longer comes about that the light transmitted throughthe interior of the substrate in the step of photolithography reflectsfrom the back of the substrate and returns to the surface to make poorthe resolution of the patterns formed. Thus, this substrate is suited tobe used in substrates for surface acoustic wave devices.

1. A lithium tantalate substrate having volume resistivity which hasbeen controlled within the range of from more than 10⁸ to less than 10¹⁰Ωcm.
 2. The lithium tantalate substrate according to claim 1, which hasa heat history of being subjected to heat treatment at a temperaturekept to from 350 to 600° C., in the state of being buried in a mixedpowder of Al and Al₂O₃. 3-4. (canceled)